iru3038 International Rectifier Corp., iru3038 Datasheet - Page 5

• Manufacturer Part Number: iru3038
• Description: Synchronous Pwm Controller For Termination Power Supply Applications
• Manufacturer: International Rectifier Corp.

APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3038,
the schematic is Figure 11 on page 12.
Output Voltage Programming
Output voltage is programmed by reference voltage and
external voltage divider. The Fb pin is the inverting input
of the error amplifier, which is referenced to the voltage
on non-inverting pin of error amplifier. For this applica-
tion, this pin (V
The output voltage is defined by using the following equa-
tion:
When an external resistor divider is connected to the
output as shown in Figure 3.
Equation (1) can be rewritten as:
If the high value feedback resistors are used, the input
bias current of the Fb pin could cause a slight increase
in output voltage. The output voltage set point can be
more accurate by using precision resistor.
Rev. 2.0
09/12/02
Figure 3 - Typical application of the IRU3038 for
V
V
I
DV
f
V
V
R
Choose R
This will result to R
OUT
S
IN
OUT
OUT
P
6
= 200KHz
= R
OUT
= V
= 5V
= 8A
= 2.5V
= V
5
= 100mV
REF
programming the output voltage.
3
P
5
P
= 1.25V
(
3 1 +
V
V
= 1KV
) is connected to reference voltage (V
REF
P
(
IRU3038
V
V
OUT
P
R
R
6
- 1
6
5
= 1KV
)
)
Fb
---(1)
V
OUT
R
R
6
5
www.irf.com
REF
).
Soft-Start Programming
The soft-start timing can be programmed by selecting
the soft-start capacitance value. The start-up time of the
converter can be calculated by using:
For a start-up time of 7.5ms, the soft-start capacitor will
be 0.1mF. Choose a ceramic capacitor at 0.1mF.
Shutdown
The converter can be shutdown by pulling the soft-start
pin below 0.5V. The control MOSFET turns off and the
synchronous MOSFET turns on during shutdown.
Boost Supply Vc
To drive the high-side switch it is necessary to supply a
gate voltage at least 4V greater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 11. The capacitor is charged up to
approximately twice the bus voltage. A capacitor in the
range of 0.1mF to 1mF is generally adequate for most
applications. In application, when a separate voltage
source is available the boost circuit can be avoided as
shown in Figure 1.
Input Capacitor Selection
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The
ripple current generated during the on time of control
MOSFET should be provided by input capacitor. The RMS
value of this ripple is expressed by:
For higher efficiency, a low ESR capacitor is recom-
mended. Choose two Poscap from Sanyo 10TPB100ML
(10V, 100mF, 55mV) with a maximum allowable ripple
current of 1.9A.
t
Where C
I
Where:
D is the Duty Cycle, D=V
I
I
For V
RMS
RMS
OUT
START
= I
is the RMS value of the input capacitor current.
is the output current for each channel.
IN
= 753Css (ms)
OUT
=5V, I
SS
is the soft-start capacitor (mF)
D3(1-D)
OUT
=8A and D=0.5, the I
OUT
---(2)
---(3)
/V
IN.
IRU3038
RMS
=4A
5